Determination of Adaptive Idle Timeout

ABSTRACT

In various embodiments, a method may be provided comprising: determining an adaptive idle timeout value based on the relationship between the number of established TCP connections to a server and the upper threshold value.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This patent application is a continuation of U.S. patent applicationSer. No. 13/009,516, filed Jan. 19, 2011 (issued as U.S. Pat. No.8,578,022 on Nov. 5, 2013), the disclosure of which is incorporatedherein, in its entirety, by reference.

TECHNICAL-FIELD

The present disclosure relates generally to preventing denial of serviceattacks and specifically to preventing denial of service attacks when aTCP connection is in an ESTAB (“Established”) state.

BACKGROUND

With today's high speed networks, the need for preventing denial ofservice (“DOS”) and distributed denial of service (“DDOS”) attacks ismuch greater. With the tools and high-end machines available today,malicious attackers may easily overwhelm an unprotected system with aDOS or DDOS attack. Thus a need exists for a system to prevent DOS andDDOS attacks when a TCP connection is in an established state.

BRIEF DESCRIPTION OF THE DRAWINGS

Many aspects of the disclosure can be better understood with referenceto the following drawings. The components in the drawings are notnecessarily to scale. Emphasis is instead placed upon clearlyillustrating the principles of the present disclosure. Moreover, in thedrawings, like references numerals designate corresponding parts throughthe several figures.

FIG. 1 is a block diagram illustrating an example environment in whichcertain embodiments of the present invention may be implemented.

FIG. 2 is a flow chart of a method for providing certain embodiments ofthe present invention.

FIG. 3 is a flow chart of a method for providing certain embodiments ofthe present invention.

FIG. 4 is a block diagram of a system including a network device.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

In various embodiments, a method may be provided comprising: determininga number of established TCP connections to a server; comparing thenumber of established TCP connections to the server to a upper thresholdvalue; and if the number of established TCP connections to the server isgreater than or equal to the upper threshold value, determine anadaptive idle timeout value based on the relationship between the numberof established TCP connections to a server and the upper thresholdvalue.

Consistent with some embodiments of the present invention, a method maybe provided comprising: scanning TCP connections to a server to identifyidle and active connections; and removing TCP connections that have beenidle for a period longer than an adaptive idle timeout, wherein theadaptive idle timeout value is determined based on the total number ofestablished TCP connections.

Consistent with some embodiments of the present invention, a system maybe provided comprising: a server, wherein the server is programmed to:determine a number of established TCP connections to a server; comparethe number of established TCP connections to the server to a upperthreshold value; and if the number of established TCP connections to theserver is greater than or equal to the upper threshold value, determinean adaptive idle timeout value based on the relationship between thenumber of established TCP connections to a server and the upperthreshold value.

FIG. 1 is a block diagram illustrating a network system 100 in whichcertain embodiments of the present invention may be implemented. Networksystem 100 may comprise, for example, a subscriber television network,though other networks are contemplated to be within the scope of thisdisclosure. Network system 100 may include a plurality of individualnetworks, such as a wireless network and/or a wired network, includingwide-area networks (WANs), local area networks (LANs), among others.

Transmitting nodes in system 100, such as server 120 and client 110 mayalso comprise other components, such as QAM modulators, routers,bridges, Internet Service Provider (ISP) facility servers, privateservers, on-demand servers, multi-media messaging servers, contentschedule servers, gateways, multiplexers, and/or transmitters, amongother equipment, components, and/or computing and communication devices.

Though system 100 is illustrated to comprise only one data transmissionnode on each side of the communications, in various embodiments, system100 may comprise any number of clients 110 and servers 120.

Embodiments of the present invention may be implemented in hardware,software, firmware, or a combination thereof (collectively orindividually also referred to herein as logic). To the extent certainembodiments, or portions thereof, are implemented in software orfirmware, executable instructions or code for performing one or moretasks are stored in memory or any other suitable computer readablemedium and executed by a suitable instruction execution system. In thecontext of this document, a computer readable medium is an electronic,magnetic, optical, or other physical device or means that can contain orstore a computer program for use by or in connection with a computerrelated system or method.

To the extent embodiments, or portions thereof, are implemented inhardware, the present invention may be implemented with any or acombination of the following technologies: a discrete logic circuit(s)having logic gates for implementing logic functions upon data signals,an application specific integrated circuit (ASIC) having appropriatecombinational logic gates, programmable hardware such as a programmablegate array(s) (PGA), a field programmable gate array (FPGA), etc.

Attacks on established TCP connections may include a Synchronization(“SYN”) Flood attack. An SYN flood is a form of denial-of-service attackin which an attacker sends a succession of SYN requests 130 to atarget's system. When client 110 attempts to start a TCP connection toserver 120, client 110 and server 120 exchange a series of messageswhich normally runs like this: 1) Client 110 may request a connection bysending a SYN message 130 to server 120. 2) Server 120 acknowledges thisrequest by sending SYN-Acknowledgement (“SYN-ACK”) 140 back to client110. 3) Client 110 may respond with an Acknowledgement (“ACK”) 150, andthe connection is established. This may be referred to as called the TCPthree-way handshake, and is typically the foundation for everyconnection established using the TCP protocol.

The SYN flood is a well known type of attack and is generally effectiveif server 120 allocates resources after receiving a SYN 130, but beforeit has received the ACK 150. Methods of SYN flooding may involve server120 not receiving the ACK 150. A malicious client 110 may skip sendingthis last ACK message 150. Alternatively, client 110 may spoof thesource IP address in the SYN 130. This may make server 120 send theSYN-ACK 140 to the falsified IP address, and client 110 will neverreceive the ACK 150. In either case, server 120 may wait for theacknowledgement 150 for some time, as simple network congestion couldpotentially be the cause of a missing ACK 150.

If these half-open connections bind resources on server 120, it may bepossible to take these resources by flooding server 120 with additionalSYN messages 130. Once all resources set aside for half-open connectionsare reserved, no new connections (legitimate or not) may be made. Thiscan result is denial of service for legitimate users. Some systems maymalfunction badly or even crash if other operating system functions arestarved of resources this way. In other scenarios so-called reflectiverouters can also be used as attackers, instead of client machines 110.

Previous attempted solutions to this problem (such as SYN-cookie) havefocused on mitigating the effects of the SYN flood attack. However, amalicious attacker may use legitimate source IP addresses to attempt tocomplete the three-way handshake and then stay idle afterwards,consuming the resources of server 120. Once the TCP three-way handshakehas been completed, server 120 resources may be held up for a longperiod of time. This period of time may be defined by an idle timeout.The idle timeout in previous systems is typically quite high once theTCP connection reaches an established state.

Notably, client 110 can establish a three-way handshake with server 120without consuming system resources of the client 110. For example, apacket crafting library, such as libnet, and a packet capturing librarylike libpcap may be employed by a malicious client 110. The malicioususer's client 110 may flood SYN packets to server 120 with differentsource points (e.g., through use of a libnet library). The maliciousclient 110 may then capture the SYN-ACK 140 from server 120 (e.g., withlibpcap) and respond back with an ACK 150 to complete the three-wayhandshake without consuming any resources on the client side. Given that64K different source ports may be used in this implementation, theresources on server 120 to handle new connections may easily be drainedby the malicious user using only a few machines.

For example, Application Control Engine (“ACE”) may be a load balancerthat may also act as a proxy for making loadbearing decisions andoffloading services like SSL. ACE may handle 256K proxied connection andthe idle timeout for each of these TCP connections in established modemay be one hour. If the Virtual IP address receives three-way handshakesfrom a few malicious hosts who intend to complete the three-wayhandshake and nothing more, all proxied connection that can be handledbe ACE may be used up by these hosts and deprive legitimate hosts ofservice.

Embodiments of the present invention employ an adaptive idle timeout tobe used to TCP connection in established mode. The adaptive idle timeoutmay decrease with an increase in the number of open connections in thesystem. The adaptive idle timeout may come into play when the number ofconnections reaches an upper threshold value. The threshold value may beconfigured by a user. The present invention to prevent DOS or DDOSattacks may be implemented on either proxies or hosts.

For example, Adaptive Idle Timeout (“AIT”) may be equal to:

-   AIT (New)=alpha*AIT(Current)—when total connections>upper threshold    value-   AIT=Initial timeout—when total connections<lower threshold value

Alpha may be a real number between 0 and 1. Alpha may further becalculable based on system conditions. The Initial timeout value mayequal the timeout configured for when the system does not have anyconnections. The lower threshold value represents a value, that onceadaptive idle timeout is in play, may determine a lower threshold ofnumber of connections below which adaptive idle timeout may be taken outof play.

The range between the upper threshold value and the maximum number ofpossible connections may be separated into n equal intervals. N may be auser configured value. Thus, when each interval is reached, the adaptiveidle timeout may be recalculated. The alpha value may be used torecalculate the adaptive idle timeout value at each interval. The alphavalue may be chosen based on the rate of increased connections once theinitial upper threshold is reached.

EXAMPLE 1 Assume:

Alpha=0.5

Lower threshold value=0.4

Upper threshold value=0.5

N=0.1

Initial timeout=0.1

With these values assumed, adaptive idle timeout comes into to play oncethe total number of connection reaches 50% of the maximum value. Theadaptive idle timeout value may then be adjusted at every 10% increasein the number of connections by reducing the adaptive idle timeout valueby half.

Sample Values:

Idle Timeout # of established connections/max # of connections 1 hour0.1 1 hour 0.5 30 minutes 0.6 15 minutes 0.7 7.5 minutes 0.8 3.75minutes 0.9

In some embodiments, a proxy may operate a scan thread that periodicallymonitors all of the connections for timeout and may clear outconnections determined to be idle. Thus, during each scan, the totalnumber of connections can be determined and the adaptive idle timeoutmay be modified accordingly so that the idle connections are clearedfrom the system faster when the system load is higher.

FIG. 2 illustrates a flow chart showing the operation of embodiments ofthe present invention. The method may begin at step 205, where thenumber of established TCP connections to a server may be determined. Asdiscussed, in some embodiments, the step of determining may beaccomplished by a proxy. Once step 205 has completed, the method mayproceed to step 215. At step 215 the number of established TCPconnections to the server may be compared to an upper threshold value.The upper threshold value may be user-configurable or based on systemparameters.

The method may then proceed to step 230 where it may be determinedwhether the number of established TCP connections to the server isgreater than or equal to the upper threshold value. If the number ofconnection does equal or exceed the upper threshold value, the methodmay proceed to step 240. At step 240, an adaptive idle timeout value maybe calculated based on the relationship between the number ofestablished TCP connections to a server and the upper threshold value.For example, the adaptive idle timeout value may be multiplied with analpha value when the number of established TCP connections to the serveris greater than or equal to the upper threshold value. In someembodiments the alpha value may be between 0 and 1. It should also benoted that the alpha value may be user-configured or automaticallycalculated based on system conditions.

Similarly, the alpha value may decrease as the number of established TCPconnections to the server increases. Alternatively, the alpha value maydecrease as the rate of newly established TCP connections to the serverincreases. In some embodiments of the present invention, the differencebetween the upper threshold value and the maximum number of availableconnections may be divided into equal intervals. The alpha value may bedecreased as the number of established TCP connections to the serverreaches each interval. The number of intervals created may be auser-configurable value.

The method may then proceed to step 250 where scanning for idle TCPconnections may take place. Again, the step of scanning may be performedby the server itself or a proxy device. Once idle TCP connections havebeen located, the method may proceed to step 260. At step 260, the idleTCP connections that have been idle for a period longer than theadaptive idle timeout value may be cleared from the system.

In some embodiments a lower threshold value may be determined The methodmay at some time proceed to step 270 and disable the adaptive idletimeout value when the number of established TCP connections to a serveris lower than the lower threshold value.

FIG. 3 illustrates a flow chart showing the operation of embodiments ofthe present invention. At step 305, TCP connections to a server may bescanned to identify idle and active connections. The method may thenproceed to step 315 where TCP connections may be removed that have beenidle for a period longer than an adaptive idle timeout, wherein theadaptive idle timeout value may be determined based on the total numberof established TCP connections.

As the total number of established TCP connections increases, the methodmay proceed to step 325, where the adaptive idle timeout value may bedecreased. In some embodiments of the present invention, the step ofdecreasing the adaptive idle timeout value only occurs when the totalnumber of established TCP connections is above an upper threshold value.Furthermore, in some embodiments a plurality of interval thresholds mayalso be defined which may trigger a modification of the adaptive idletimeout value.

Embodiments of the present invention may provide a mechanism that mayprovide a mechanism which can mitigate attacks where the maliciousclient(s) does not use spoofed IP addresses, but instead use their ownIP addresses and thus may respond to respond to keepalives sent by anend host. When under attack, the use of keepalives may be unwanted sincethe malicious client may respond back to the keepalives and resultantlykeep the connection open and consume the host's resources.

As such, embodiments of the present invention may use adaptively timeoutsuch connections “silently” to free up connections that are idle as notrigger may be provided back to the malicious client. During an attack,the connections need to be timed out silently to avoid providinginformation to the malicious clients.

Embodiments of the present invention may incorporate reset timeouts tonotify genuine clients to close the connection when the host times outthe connection on its end. As a result, the client connection shouldavoid getting hung up. Similarly, when dealing with a genuine client,keepalives may be a good way to know if the remote end of the connectionis no longer active and that the host needs to free up these additionalresources. However, when dealing with a malicious client, using a resettimeout and keepalives may again be problematic as it may providetriggers for the malicious client to reopen the connection.

Accordingly, embodiments of the present invention may distinguishbetween “peace time” and “attack time”. During peace time, resettimeouts and keepalives may be used. During attack time, silent timeoutswith no keepalives may be employed. Attack time may be a user-definedparameter based on the number of active open connections and/or thenumber of connections open for each source IP address.

For example, attack time may be defined as the period of time when thenumber of currently open connections exceeds an upper threshold of totalconnections that can be opened in the system. Alternatively, attack timemay be defined as the period of time when the number of currently openconnections exceeds an upper threshold on a per-source-IP address basis.

Similarly, peace time may be defined as the period of time when thenumber of currently open connections is less than a lower threshold oftotal connections that can be opened in the system. Alternatively, peacetime may be defined as the period of time when the number of currentlyopen connections is less than a lower threshold on a per-source-IPaddress basis. It should be understood that the definitions of peacetime and attack time may be configurable based on a user selection ofparameters. Accordingly, when embodiments of the present inventionemploy a per-source-IP address threshold, the adaptive idle timeoutvalue may come into play only for connections originated by a source IPaddress which has passed the per-source-IP address upper threshold.

The systems and methods described in FIGS. 1-3 may be implemented usinga network device 400 as described in more detail below with respect toFIG. 4. The network device may comprise, for example, but is not limitedto, any transmitting network node device depicted in FIG. 1 such asserver 120.

FIG. 4 is a block diagram of a system including network device 400.Consistent with embodiments of the present invention, the aforementionedmemory storage and processing unit may be implemented in a networkdevice, such as network device 400 of FIG. 4. Any suitable combinationof hardware, software, or firmware may be used to implement the memorystorage and processing unit. For example, the memory storage andprocessing unit may be implemented with network device 400 or any ofother network devices 418, in combination with network device 400. Theaforementioned system, device, and processors are examples and othersystems, devices, and processors may comprise the aforementioned memorystorage and processing unit, consistent with embodiments of the presentinvention. Furthermore, network device 400 may comprise an operatingenvironment for system 100 as described above. System 100 may operate inother environments and is not limited to network device 400.

With reference to FIG. 4, a system consistent with embodiments of thepresent invention may include a network device, such as network device400. In a basic configuration, network device 400 may include at leastone processing unit 402, a secure processing unit for decryption 420,and a system memory 404. Depending on the configuration and type ofnetwork device, system memory 404 may comprise, but is not limited to,volatile (e.g., random access memory (RAM)), non-volatile (e.g.,read-only memory (ROM)), flash memory, or any combination. System memory404 may include operating system 405, one or more programming modules406, and may include program data 407. Operating system 405, forexample, may be suitable for controlling network device 400's operation.Furthermore, embodiments of the present invention may be practiced inconjunction with a graphics library, other operating systems, or anyother application program and is not limited to any particularapplication or system. This basic configuration is illustrated in FIG. 4by those components within a dashed line 408.

Network device 400 may have additional features or functionality. Forexample, network device 400 may also include additional data storagedevices (removable and/or non-removable) such as, for example, magneticdisks, optical disks, or tape. Such additional storage is illustrated inFIG. 4 by a removable storage 409 and a non-removable storage 410.Computer storage media may include volatile and nonvolatile, removableand non-removable media implemented in any method or technology forstorage of information, such as computer readable instructions, datastructures, program modules, or other data. System memory 404, removablestorage 409, and non-removable storage 410 are all computer storagemedia examples (i.e., memory storage.) Computer storage media mayinclude, but is not limited to, RAM, ROM, electrically erasableread-only memory (EEPROM), flash memory or other memory technology,CD-ROM, digital versatile disks (DVD) or other optical storage, magneticcassettes, magnetic tape, magnetic disk storage or other magneticstorage devices, or any other medium which can be used to storeinformation and which can be accessed by network device 400. Any suchcomputer storage media may be part of device 400. Network device 400 mayalso have input device(s) 412 such as a keyboard, a mouse, a pen, asound input device, a touch input device, etc. Output device(s) 414 suchas a display, speakers, a printer, etc. may also be included. Theaforementioned devices are examples and others may be used.

Network device 400 may also contain a communication connection 416 thatmay allow device 400 to communicate with other network devices 418, suchas over a network in a distributed network environment, for example, anintranet or the Internet. Communication connection 416 is one example ofcommunication media. Communication media may typically be embodied bycomputer readable instructions, data structures, program modules, orother data in a modulated data signal, such as a carrier wave or othertransport mechanism, and includes any information delivery media. Theterm “modulated data signal” may describe a signal that has one or morecharacteristics set or changed in such a manner as to encode informationin the signal. By way of example, and not limitation, communicationmedia may include wired media such as a wired network or direct-wiredconnection, and wireless media such as acoustic, radio frequency (RF),infrared, and other wireless media. The term computer readable media asused herein may include both storage media and communication media.

As stated above, a number of program modules and data files may bestored in system memory 404, including operating system 405. Whileexecuting on processing unit 402 or secure processing unit fordecryption 420, programming modules 406 may perform processes including,for example, one or more method 200's stages as described above. Theaforementioned process is an example; processing unit 402 and secureprocessing unit for decryption 420 may perform other processes.

Generally, consistent with embodiments of this invention, programmodules may include routines, programs, components, data structures, andother types of structures that may perform particular tasks or that mayimplement particular abstract data types. Moreover, embodiments may bepracticed with other computer system configurations, including hand-helddevices, multiprocessor systems, microprocessor-based or programmableconsumer electronics, minicomputers, mainframe computers, and the like.Embodiments of the present invention may also be practiced indistributed network environments where tasks are performed by remoteprocessing devices that are linked through a communications network. Ina distributed network environment, program modules may be located inboth local and remote memory storage devices.

Furthermore, embodiments of the present invention may be practiced in anelectrical circuit comprising discrete electronic elements, packaged orintegrated electronic chips containing logic gates, a circuit utilizinga microprocessor, or on a single chip containing electronic elements ormicroprocessors. Embodiments may also be practiced using othertechnologies capable of performing logical operations such as, forexample, AND, OR, and NOT, including but not limited to mechanical,optical, fluidic, and quantum technologies. In addition, embodiments ofthe invention may be practiced within a general purpose computer or inany other circuits or systems.

Embodiments of the present invention, for example, may be implemented asa computer process (method), a network system, or as an article ofmanufacture, such as a computer program product or computer readablemedia. The computer program product may be a computer storage mediareadable by a computer system and encoding a computer program ofinstructions for executing a computer process. The computer programproduct may also be a propagated signal on a carrier readable by anetwork system and encoding a computer program of instructions forexecuting a computer process. Accordingly, aspects of the presentinvention may be embodied in hardware and/or in software (includingfirmware, resident software, micro-code, etc.). In other words,embodiments of the present invention may take the form of a computerprogram product on a computer-usable or computer-readable storage mediumhaving computer-usable or computer-readable program code embodied in themedium for use by or in connection with an instruction execution system.A computer-usable or computer-readable medium may be any medium that cancontain, store, communicate, propagate, or transport the program for useby or in connection with the instruction execution system, apparatus, ordevice.

The computer-usable or computer-readable medium may be, for example butnot limited to, an electronic, magnetic, optical, electromagnetic,infrared, or semiconductor system, apparatus, device, or propagationmedium. More specific computer-readable medium examples (anon-exhaustive list), the computer-readable medium may include thefollowing: an electrical connection having one or more wires, a portablecomputer diskette, a random access memory (RAM), a read-only memory(ROM), an erasable programmable read-only memory (EPROM or Flashmemory), an optical fiber, and a portable compact disc read-only memory(CD-ROM). Note that the computer-usable or computer-readable mediumcould even be paper or another suitable medium upon which the program isprinted, as the program can be electronically captured, via, forinstance, optical scanning of the paper or other medium, then compiled,interpreted, or otherwise processed in a suitable manner, if necessary,and then stored in a computer memory.

Embodiments of the present invention, for example, are described abovewith reference to block diagrams and/or operational illustrations ofmethods, systems, and computer program products according to embodimentsof the present invention. The functions/acts noted in the blocks mayoccur out of the order as shown in any flowchart. For example, twoblocks shown in succession may in fact be executed substantiallyconcurrently or the blocks may sometimes be executed in the reverseorder, depending upon the functionality/acts involved.

While certain embodiments of the present invention have been described,other embodiments may exist. Furthermore, although embodiments have beendescribed as being associated with data stored in memory and otherstorage mediums, data can also be stored on or read from other types ofcomputer-readable media, such as secondary storage devices, like harddisks, floppy disks, or a CD-ROM, a carrier wave from the Internet, orother forms of RAM or ROM. Further, the disclosed methods' stages may bemodified in any manner, including by reordering stages and/or insertingor deleting stages, without departing from the invention.

While the specification includes examples, the invention's scope isindicated by the following claims. Furthermore, while the specificationhas been described in language specific to structural features and/ormethodological acts, the claims are not limited to the features or actsdescribed above. Rather, the specific features and acts described aboveare disclosed as example for embodiments of the present invention.

At least the following is claimed:
 1. A method comprising: determine anadaptive idle timeout value by multiplying a previous adaptive idletimeout value with an alpha value.
 2. The method of claim 1, furthercomprising: clearing idle TCP connections that have been idle for aperiod longer than the adaptive idle timeout value.
 3. The method ofclaim 1, further comprising: decreasing the adaptive idle timeout bymultiplication with an alpha value when the number of established TCPconnections to the server is greater than or equal to an upper thresholdvalue.
 4. The method of claim 3, wherein the upper threshold value is auser-configurable value.
 5. The method of claim 3, wherein the alphavalue is automatically calculated based on system conditions.
 6. Themethod of claim 5, further comprising: disabling the adaptive idletimeout value when the number of established TCP connections to a serveris lower than a lower threshold value.
 7. The method of claim 5, furthercomprising: decreasing the alpha value as the number of established TCPconnections to the server increases.
 8. The method of claim 5, furthercomprising: increasing the alpha value as the rate of newly establishedTCP connections to the server decreases.
 9. The method of claim 7,further comprising: decreasing the alpha value as the number ofestablished TCP connections to the server reaches an interval betweenthe upper threshold value and the maximum number of availableconnections.
 10. The method of claim 9, wherein a number of intervals isa user-configurable value.
 11. A method comprising: removing TCPconnections that have been idle for a period longer than an adaptiveidle timeout determined based on whether the total number of establishedTCP connections is greater than an upper threshold value by multiplyinga previous adaptive idle timeout value with an alpha value.
 12. Themethod of claim 11, further comprising: decreasing the adaptive idletimeout value as the total number of established TCP connectionsincreases.
 13. The method of claim 12, further comprising: determiningwhether the total number of established TCP connections is above anupper threshold value.
 14. The method of claim 12, further comprising:triggering a modification of the adaptive idle timeout value at adetermined interval threshold.
 15. A system comprising: a server device,wherein the server device is programmed to: determine an adaptive idletimeout value based on the relationship between the number ofestablished TCP connections to a server and the upper threshold value bymultiplying a previous adaptive idle timeout value with an alpha value.16. The system of claim 15, wherein the server device is a proxy device.17. The system of claim 15, further comprising a scanner programmed todetermine a time which each established TCP connection has been idle.18. The system of claim 16, wherein the proxy device is programmed toremove TCP connections which have been idle for a period longer than theadaptive idle timeout value.
 19. The system of claim 15, wherein theserver device is further programmed to: decrease the adaptive idletimeout when the number of established TCP connections to the server isgreater than or equal to the upper threshold value.
 20. The system ofclaim 19, wherein the server device is further programmed to: decreasethe alpha value as the rate of newly established TCP connection to theserver increases.